fix: web UI and src.common bugs (wifi wrong-password, plugin icon, starlark toggle, API caching, scroll/logo/font helpers) (#646)

- wifi: keep the "wrong_password:" prefix through the restore/AP fallback so
  the UI's incorrect-password prompt fires again.
- /plugins/installed returns the manifest's icon (string only).
- /starlark/apps/<id>/toggle coerces `enabled` and delegates to
  _toggle_starlark_app (disk before memory, no KeyError, "false" is false).
- /api/v3/ JSON GETs are sent Cache-Control: no-store; non-JSON keeps 5s.
- ScrollHelper.set_scrolling_image converts non-RGB input (alpha onto black);
  create/set_scrolling_image reset last_update_time like reset_scroll.
- LogoHelper backs off a failed download per path for
  MISSING_LOGO_RECHECK_SECONDS; cleared on invalidate/clear_cache.
- refresh_placeholder_timestamp saves atomically.
- FontManager.clear_cache / _clear_plugin_font_cache bump cache_generation.
- Odds manager: per-game logs to DEBUG; JSON decode error caught before
  RequestException (same cooldown).
- element_style mangled continuations; startup validator skips null plugin
  blocks and reuses the controller's discovery.
- src/common/README lists frame_timing, json_body, render_gate.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/test_scroll_helper_image_entry.py

@@ -0,0 +1,87 @@
+"""ScrollHelper: what a new scrolling image does on the way in.
+
+- set_scrolling_image cached a non-RGB image as-is, and every frame is cut
+  with Image.frombytes('RGB', ...): an RGBA strip came out as garbage and an
+  L strip raised.
+- Only reset_scroll() reset last_update_time. In time-based mode the first
+  update after a new image (e.g. the plugin coming back on screen) advanced
+  the position by the whole idle gap, skipping the start of the content.
+"""
+
+import time
+
+import pytest
+from PIL import Image
+
+from src.common.scroll_helper import ScrollHelper
+
+W, H = 16, 8
+
+
+@pytest.fixture
+def helper():
+    return ScrollHelper(display_width=W, display_height=H)
+
+
+class TestNonRgbImages:
+    def test_rgba_is_composited_onto_black(self, helper):
+        img = Image.new('RGBA', (40, H), (0, 0, 0, 0))
+        for x in range(10):
+            for y in range(H):
+                img.putpixel((x, y), (255, 0, 0, 255))
+        # Hidden colour under full transparency must not leak through.
+        img.putpixel((20, 0), (0, 255, 0, 0))
+        helper.set_scrolling_image(img)
+        assert helper.cached_image.mode == 'RGB'
+        frame = helper.get_visible_portion()
+        assert frame.size == (W, H)
+        assert frame.getpixel((0, 0)) == (255, 0, 0)
+        assert frame.getpixel((12, 0)) == (0, 0, 0)
+        helper.scroll_position = 20.0
+        assert helper.get_visible_portion().getpixel((0, 0)) == (0, 0, 0)
+
+    def test_greyscale_does_not_raise(self, helper):
+        helper.set_scrolling_image(Image.new('L', (40, H), 200))
+        frame = helper.get_visible_portion()
+        assert frame.getpixel((0, 0)) == (200, 200, 200)
+
+    def test_palette_with_transparency(self, helper):
+        img = Image.new('P', (40, H), 1)
+        img.putpalette([0, 0, 0, 0, 0, 255] + [0] * 762)
+        img.info['transparency'] = 1
+        helper.set_scrolling_image(img)
+        assert helper.get_visible_portion().getpixel((0, 0)) == (0, 0, 0)
+
+    def test_an_rgb_image_is_kept_as_the_same_object(self, helper):
+        img = Image.new('RGB', (40, H), (1, 2, 3))
+        helper.set_scrolling_image(img)
+        assert helper.cached_image is img
+
+
+class TestNoJumpAfterIdle:
+    def _idle_then_new_image(self, helper, install):
+        helper.scroll_speed = 100.0  # px/s, time-based
+        install(helper)
+        helper.update_scroll_position()
+        # Off screen for 10 seconds, then a fresh image.
+        helper.last_update_time = time.time() - 10.0
+        install(helper)
+        helper.update_scroll_position()
+        return helper.scroll_position
+
+    def test_set_scrolling_image(self, helper):
+        pos = self._idle_then_new_image(
+            helper, lambda h: h.set_scrolling_image(Image.new('RGB', (5000, H))))
+        assert pos < 50, f"jumped {pos}px on the first update of a new image"
+
+    def test_create_scrolling_image(self, helper):
+        pos = self._idle_then_new_image(
+            helper, lambda h: h.create_scrolling_image([Image.new('RGB', (5000, H))]))
+        assert pos < 50, f"jumped {pos}px on the first update of a new image"
+
+    def test_fixed_step_mode_still_steps_exactly(self, helper):
+        helper.set_pixels_per_frame(2)
+        helper.set_scrolling_image(Image.new('RGB', (5000, H)))
+        helper.update_scroll_position()
+        helper.update_scroll_position()
+        assert helper.scroll_position == 4.0